12 people have been killed and 31 injured in a bus crash in Croatia. 

The bus, which had driven from Poland, was carrying pilgrims when it reportedly slipped off a main road and crashed near Varazdin in northwestern Croatia on Saturday morning.

The passengers were adult pilgrims who were travelling to Medjugorje, a Roman Catholic shrine in southern Bosnia, Croatian Interior Minister Davor Bozinovic told reporters. 

Police, firefighters and medical teams arrived on site at 5:40 a.m. near Breznicki Hum in the direction of the capital Zagreb, the police stated.

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"We have 43 injured people, of them 12 deceased," said Maja Grba-Bujevic, the director of the Croatian Emergency Medicare Institute.

"All the victims are Polish citizens - we can at this point confirm this," a Polish Foreign Ministry spokesman told Polish broadcaster TVN24. "The bus has Warsaw registration plates."

An investigation into the cause of the accident has begun according to Croatian authorities.